Life-threatening hypotension associated with the rapid administration of certain batches of plasma protein fraction to surgical patients is very probably caused by the rapid generation of bradykinin. Implicated batches contain an activator of prokallikrein that is derived from Hageman factor. Highly purified human renin liberates a substance from highly purified human kininogen which contracts the isolated rat uterus. Neither human nor hog renin activate human plasma prokallikrein. The radioimmunoassay for kinins has been further investigated and the cross reactivity of kinin degradation products, kininogens and naturally occurring kinins has been determined. The assay of urokininogen has been modified.